Glossary

Glossary of IP telephony and voice

Short, clear definitions of the terms that come up when contracting IP telephony, SIP trunking, a cloud PBX or SMS. Written so anyone in your company understands what we're talking about.

Updated: 17 October 2026

VoIP (IP telephony)

Technology that carries voice calls over the internet instead of analogue lines. It is the basis of business VoIP.

SIP

The standard protocol that sets up and controls voice-over-IP calls. It is the "language" that PBXs, phones and carriers use to talk to each other.

SIP trunk

A link connecting a phone system to the public network over the internet. It lets you call and receive without physical lines. See SIP trunking.

PBX / cloud PBX

The system that manages a company's extensions, menus and queues. When it lives in the cloud it is called a cloud PBX. See miPBX.

IVR

Voice attendant menu ("press 1 for sales"). It routes each call to the right destination.

DID

A direct phone number assigned to a company or extension, so people can reach a specific number without going through reception.

Number portability

Moving your numbers from one carrier to another without changing them. Your customers keep dialling the same.

Concurrency

The number of simultaneous calls your operation can carry. A SIP trunk scales by concurrency, not by physical lines.

OTP

A one-time password sent, for example, by SMS to verify identity in a login or a transaction.

ASR and ACD

Voice-traffic indicators. ASR (Answer-Seizure Ratio) measures the share of calls answered; ACD (Average Call Duration) measures the average length of calls.

UCaaS and CCaaS

Unified communications as a service (UCaaS) and contact center as a service (CCaaS): telephony and customer service delivered from the cloud, by subscription.

Frequently asked questions